The preparation of heterophasic copolymers via the sequential polymerization is sometimes referred to as the “in situ blending of polymers.”. According to this technique, a relatively high crystalline propylene polymer is prepared in a first polymerization reactor and then transferred to a successive polymerization reactor, wherein a low crystallinity elastomeric component (for instance, a propylene-ethylene copolymer) is formed.
In this process, where each reactor can work at different polymerization conditions, in terms of catalyst, pressure, temperature, amounts of comonomer(s) and molecular weight regulator(s), the tailoring of the process conditions allows to produce a wide range of heterophasic propylene copolymers, as well as different concentrations of semicrystalline component and low crystallinity elastomeric component. Processes of this type are described, for example, in EP Pat. Doc. No. 640649 and WIPO Pat. Doc. No. WO2010/146074 where the catalyst system is formed by pre-contacting, before the first polymerization step, a solid catalyst component of the Ziegler-Natta type, in which a phthalate or a mixture of 1,3-diether and succinate are used as internal donor, a trialkyl-aluminum co-catalyst and an alkyl alkoxysilane as external electron donor to improve stereospecificity. In both cases, the precontacting temperature is set preferably in the range 0-30° C. In EP Pat. Doc. No. 640649 the precontacting temperature is 0° C. while in WIPO Pat. Doc. No. WO2010/146074 a temperature of 25° C. is used.
When the sequential polymerization process is directed to the preparation of heterophasic copolymers with a relevant amount of low crystallinity elastomeric polymer, the porosity of the relatively high crystallinity polymer matrix plays an important role.
As a general rule, the higher is the porosity of the polymer matrix produced in the first step, the higher is the amount of elastomeric component that can be incorporated, within the matrix, in the second polymerization step.
On the other hand, if the porosity of the matrix is poor, the presence of an excessive amount of elastomeric polymer fraction on the surface of the particles considerably increases the tackiness of the particles giving raise to agglomeration phenomena, which in turn can cause reactor walls sheeting, plugging or even clogging.
An important macroscopic measurement of the polymer porosity is given by the polymer bulk density. The bulk density or apparent density is the mass per unit of volume of a material, including voids inherent in the material of interest. In case of polymer particles of regular morphology, relatively low values of bulk density indicate a relatively high porosity of the polymer powder. Thus, at least for certain applications it would be desired to produce in the first polymerization step a propylene polymer endowed with both higher porosity (lower bulk density) and high crystallinity.
As described in WIPO Pat. Doc. No. WO2008/015113, it is possible to modulate the porosity of the polymer matrix produced in the first polymerization step by careful selection of catalyst pre-contact and polymerization step. In particular, contacting the Z—N catalyst component (including a donor selected from the group consisting of, among others, phthalates, succinates and ethers) with an alkyl-Al compound, an external donor compound, optionally in the presence of propylene, at a temperature from 5° C. to 30° C. and a weight ratio propylene/(catalytic component) ranging from 0 to 2.0 in order to prepare a high crystallinity polymer matrix having a value of bulk density lower than 0.40 g/cm3 is possible. As further described therein, the bulk density of the semi-crystalline matrix may be decreased by setting the pre-contact temperature in the higher end of the range 5-30° C. WIPO Pat. WO2010/146074 further describes an improvement in polymer porosity with respect to the 0° C. pre-contact temperature of EP Pat. Doc. No. 640649. However, the working examples of the cited reference do not explore the whole range of pre-contact temperatures but limit the treatment in the range of 15-25° C. By comparison of Examples 1 and 2 it is possible to see that the increase of pre-contact temperature involves a decrease of polymer bulk density (therefore higher porosity) but also a decrease of catalyst stereospecificity demonstrated by the higher amount of xylene soluble matter. It is therefore clear that the 30° C. upper limit for the pre-contact temperature has its technical basis on the necessary compromise between high porosity and high crystallinity. In fact, the comparative runs carried out by the applicant confirm that by pre-contacting at 30° C. a Z—N catalyst component containing phthalates, an aluminum alkyl and an alkoxysilane, the decrease in stereospecificity is even more pronounced.
In view of the above, it has been very surprising to discover that with a specific catalyst containing both diethers and succinates as internal donors, higher pre-contact temperatures can be used without substantially observing decrease of stereospecificity.